The purpose of this research is to develop a model for the induction of choriocarcinoma from normal rat trophoblast. The inductive procedure is based on a number of techniques which include fetectomy, prolongation of the luteal phase and administration of a carcinogenic drug. Fetectomy arrests the normal differentiation of the placenta. If performed before gestation day 12 the placenta persists composed almost entirely of basal zone trophoblast in which the percentage of trophoblastic giant cells also increases with time. This is the substratum for induction of choriocarcinoma by carcinogenic drugs. The time necessary for induction is assured by hormonal prolongation of the life of the basal zone trophoblast. Progression from normal to malignant cells will be monitored by studies of the endocrine functions of the persisting trophoblast, ontogeny of both peptide and steroid hormone receptors and alterations in the expression of the genome. Correlation of these data should provide probes for future study as well as aid the delineation of factors associated with induction of choriocarcinoma. The development of transplantable cloned cell lines, particularly induced choriocarcinoma, will provide a source of stable, permanent choriocarcinoma cells for necessary in vitro studies.